This application is related to concurrently filed application Ser. No. 876,229 entitled "Bit Interleaved Multiplexer System Providing Byte Synchronization for Communicating Apparatuses", which is hereby incorporated by reference herein and which is assigned to the assignee herein.
The present invention relates to time-division-multiplexed (TDM) bit interleaved telecommunication systems. More particularly the invention relates to apparatuses and methods for efficiently allocating bandwidth to data, control, synchronization and intermultiplexer communication information, and for providing flexible data rates and control rates with high utilization efficiency.
In a typical TDM system, a transmit frame of a multiplexer samples signals of relatively low frequency from various data sources or channels and interleaves them with one another to form an aggregate data stream that is transmitted by a high speed aggregate channel to the receive frame of a remote demultiplexer. Ordinarily, the TDM transmit frame inserts the signals representative of a single bit or a single character in a single time slot in the aggregate data stream and interleaves the signals from different channels on a bit-by-bit or character-by-character basis so that adjacent time slots contain signals from different channels. At the receive frame, the individual bits or characters are separated from one another and allocated to various low frequency data channels similar to those on the transmit end.
To permit proper TDM communication, prior art systems have the transmit frame interleave the signals from the various data channels in accordance with a fixed schedule (frame) which repeats endlessly and the receive frame uses the same schedule to decode the data stream. In addition to the data signals from the channels, the frame ordinarily interleaves control signals from the channels as well as frame synchronization signals and system control signals. Typically, the synchronization and system control signals are referred to as "overhead" and arranged to take up a small portion of the total frame.
In arranging a frame which permits TDM communication, different framing algorithm schemes have been used. The algorithms are limited by the fact that every channel has to be sampled at least once in every frame and as often as required to ensure that all the data from the channel is interleaved in the aggregate data stream. In addition, a set of overhead signals must be transmitted for each frame, and control signals for the channels may also be included in the TDM communication. Since every channel has to be sampled at least once in every frame, the speed of the slowest channel in the frame determines the maximum number of frames that can be transmitted per unit time without wasting the bandwidth of the aggregate channel. In addition, since the synchronization signals transmitted in each frame are approximately the same regardless of the length of the frame, there is some incentive to use very long frames in order to decrease the overhead and increase data transmission. As a result, the frames tend to be quite along, inflexible, and difficult to configure, and memory of considerable size is required to store the frame.
To simplify the frame configuration, it is has been shown that the data channels may be sampled in a fixed pattern which is repeated numerous times within each frame. Each such cycle of repetition is called a subframe. Likewise, as disclosed in U.S. Pat. No. 4,460,993 to Hampton et al. which is assigned to the assignee herein, a frame can be divided into repeating and non-repeating portions with the subframe being considered as the repeating portion. In U.S. Pat. No. 4,460,993, a framing algorithm is disclosed which assigns data, control, and overhead "selects" (signals used by a multiplexer to select channels) into repeating and non-repeating sections. Thus, the channel data rates are sorted and ordered by their rates. The greatest common denominator of the data rates is found and called the "frame rate", and the data rate divided by the frame rate determines the number of selects required for each data channel per frame. The aggregate rate divided by the frame rate gives the number of selects required for one frame. A subframe rate is defined by dividing the number of selects required for one frame by five hundred and twelve and rounding up. In assigning locations in the repeating and nonrepeating sections of the frame according to U.S. Pat. No. 4,460,993, the selects required per frame for each data channel are divided by the subframe rate. The integer value obtained determines the number of slots of the subframe (repeating section) that are to be taken by the channel. The remainder of the selects required for the data channel determines the number of slots in the nonrepeating frame portion to be allocated to the channel. System and channel control information is then inserted into remaining slots in the nonrepeating portion of the frame.
While the framing algorithm of U.S. Pat. No. 4,460,993 provided a great improvement in the art in enhancing efficiency, it still suffers from a certain lack of flexibility. Thus, if it is desired that more than a minimum of control information is to be transmitted over the aggregate, but the sum of the data rates plus the minimum controls and overhead rates approaches the aggregate rate, a data channel must be dropped to accommodate the controls. This is in contrast to previous framing schemes which assigned fixed amounts of control and overhead into the frame whether needed or not, thereby limiting the throughput of the data.